Modern vehicles comprise in many cases electronic parking brake systems, in particular systems of the type where, in response to a request by the driver, electric actuators actuate a tensioning device on the friction brakes of the rear axle. Owing to the great concern for safety, electric operated parking brakes of this type must comply with different judicial and accordingly legal requirements.
In the case of a parking brake operating switch that is embodied as an operating switch and its connection to an evaluating unit as a control unit that controls the parking brake it is therefore necessary that individual faults such as short circuits in individual lines with respect to each other or with respect to one of the supply voltages, interruptions in a line and defects in the switching contacts are detected in a reliable manner as early as possible, irrespective of the parking brake being actuated. Fault tolerance with respect to an individual fault is also necessary in some cases since the parking brake must be actuated in response to a request by the driver even in the presence of an individual fault.
EP 1 128 999 B2 discloses a device and a method for controlling an electric operated parking brake, wherein an input device comprises an electric switching means having a plurality of switch positions, which electric switching means is coupled to switches that are embodied in a redundant manner and are arranged in parallel-functioning switch circuits having their own energy supply and ground connection. The input device emits at least two redundant output signals in each switch position, wherein in addition in each case at least two signal lines are allocated to each switch circuit. At least eight lines are disadvantageously necessary for the connection between the input device and control device.
DE102008015910 discloses a method for testing the functionality of a switching unit that comprises a plurality of switches and at least one preferably internal control unit and can be connected by way of at least one interface unit to an external control device. In this case, the functionality of the switching unit is tested using an internal diagnostic routine that is embodied in the control device or using an external diagnostic route that is embodied in the external control device, wherein the diagnostic routines generate at least one test signal that is transmitted to at least one of the switches, and input signals that are received from the switches are evaluated in the control unit. In that system, the switching unit is expensive owing to the internal control unit, at least five lines would be necessary when using an external control unit.
WO2009/0658264 discloses a device for electrically actuating a system where safety is of great concern and which device comprises at least two connections, at least one switch and an operating element by means of which the at least two operating modes can be selected for the system, by means of which device switch positions of the at least one switch are determined. By using at least one current direction element, it is achieved that current flows in a uni-direction between the at least two connections, the current direction for a first operating mode being in the direction from the first terminal to the second terminal and for a further operating mode in the reverse direction from the second terminal to the first terminal. It is therefore necessary to have at least four terminals and accordingly four lines between the switching unit and the control device in order to provide a tolerance with respect to an individual fault.
In addition to the lines necessary in order for a request to be made that the parking brake is to be actuated, it is often the case that one or two further lines are routed to the parking brake operating switch, which lines render it possible to control a signal light in or on the switch. As a consequence, the operating mode of the parking brake (applied or released) can be displayed and/or the switch illuminated.
It follows from this, that an operating switch for a parking brake having the switch positions Apply/Idle/Release and an illuminated display requires in accordance with the prior art at least five lines between the parking brake control device and parking brake operating switch.
The object of the present invention is therefore to provide a method for detecting a fault of an operating switch of the type mentioned in the introduction, with which method an individual fault can be detected in a reliable manner as early as possible, preferably without a fault of this type having a negative influence on the detection of the switching mode and by means of a method of this type in accordance with the invention it is rendered possible to achieve an operating switch having a smallest possible number of connection lines between the operating switch and control device. Furthermore, it is the object of the invention to provide an operating switch for performing the method in accordance with the invention.
The first object mentioned is achieved by means of a method having the features described herein.